For the basic dough: Put flour, butter in flakes, icing sugar and eggs in a bowl. First work into a smooth dough with the dough hook of the hand mixer, then with your hands.
For the Frankfurter Kränzchen: Roll out the dough on a lightly floured work surface, approx. 7 mm thin. Cut out 42 rings (5 cm Ø each) with a round stamp cutter. Knead the dough again and again and roll out again.
Mix 1 egg yolk and milk. Brush the 14 rings with it and sprinkle with hazelnut brittle. Put all rings on 2 baking trays lined with baking paper. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) one after the other for 10-12 minutes. Let the cookies cool down.
Heat the jam in a small pot and stir until smooth. For the buttercream, whisk butter and icing sugar in a bowl with the whisks of the hand mixer until firm. Pour the mixture into a piping bag with star-shaped spout. Spread 14 rings with jam. Spray buttercream onto 14 rings and add some jam to each ring. Place 3 cookies on top of each other (first with jam only, then with the buttercream and finally 1 cookie with hazelnut brittle). Store cookies in the refrigerator.